Why.  The region field of GdkEventExpose allows you to redraw less than the traditional GdkEventRegion.area.

In early GTK+ versions, the GdkEventExpose structure only had an area field to let you determine the region that you needed to redraw. In current GTK+, this field still exists for compatibility and as a simple interface. However, there is also a region field which contains a fine-grained region. The area field is simply the bounding rectangle of the region.

Widgets that are very expensive to re-render, such as an image editor, may prefer to use the GdkEventExpose.region field to paint as little as possible. Widgets that just use a few drawing primitives, such as labels and buttons, may prefer to use the traditional GdkEventExpose.area field for simplicity.

Regions have an internal representation that is accessible as a list of rectangles. To turn the GdkEventExpose.region field into such a list, use gdk_region_get_rectangles():

static gboolean
my_widget_expose_event_handler (GtkWidget *widget, GdkEventExpose *event)
{
  GdkRectangle *rects;
  int n_rects;
  int i;

  gdk_region_get_rectangles (event->region, &rects, &n_rects);

  for (i = 0; i < n_rects; i++)
    {
      /* Repaint rectangle: (rects[i].x, rects[i].y),
       *                    (rects[i].width, rects[i].height)
       */
    }

  g_free (rects);

  return FALSE;
}